1. Field of the Invention
The present invention relates to the field of data storage devices. More specifically, embodiments of the present invention relate to an improved servo burst pattern for a hard disk drive.
2. Related Art
A disk storage system, such as a magnetic hard disk drive (HDD), uses one or more disks or “platters” as a data recording medium. The HDD records data on the disk by use of a head which can also reproduce data from the disk.
Increased levels of storage capacity in floppy and hard disk drives are a direct result of the higher track densities possible with voice-coil and other types of servo positioners used today. Previously, low track density disk drives were able to achieve satisfactory head positioning with lead screw and stepper motor mechanisms. However, when track densities are so great that the mechanical error of a lead screw-stepper motor combination is significant compared to track-to-track spacing; an embedded servo-pattern became necessary so that the position of the head can be determined from the signals it reads off of the storage medium.
As a result, conventional hard disk manufacturing techniques often include writing servo-patterns (or servo “bursts”) on the media of a head disk assembly (HDA) with a specialized servowriter instrument. Conventional disk drive assemblies typically include one or more disks, which include a plurality of concentric tracks that are radially displaced from each other on the surface of the disk for storing data. During disk fabrication, servo data is written on the disk by a servowriting process to delineate the centerlines of the tracks. During subsequent disk operations, the servo data is also read by a read/write head to provide information regarding the position of the head with respect to the track. The head position information enables a servo controller to re-align the head over a track when position errors are detected. Conventional servo-patterns typically comprise short bursts, very precisely located offset from a track's center line, on either side. Each track has a preset number of data sectors arranged between the servo sectors and user data is recorded in the data sector. The servo information may include cylinder data (track address code) used for the seeking operation and servo burst data.
The servo bursts are written in a sector header area, and can be used to find the center line of a track. Staying on track center is required during both reading and writing. These servo-pattern areas allow a head to follow a track center line around a disk, even when the track is out of round, as can occur with spindle wobble, disk slip and/or thermal expansion.
More specifically, the bursts are generally, but not required to be, located in a trajectory within a track. The servo burst data is constructed by a plurality of burst patterns for deriving a positional error (position data) of the head in a range of the target head or in a range to the adjacent track after the head is moved to a position near the target track by the seeking operation. The processor of the head positioning control circuit converts the amplitude of a position signal waveform (PES) obtained when the head reads the burst pattern into digital data and effects the processing operation for deriving the positional error by using the digital data. As technology advances provide smaller disk drives, and increased track densities, the placement of servo-patterns becomes crucial for successful hard drive designs.
Servo-patterns are conventionally written by dedicated, external servowriting equipment, and typically involve the use of large granite blocks to support the disk drive and quiet outside vibration effects. An auxiliary clock head is inserted onto the surface of the recording disk and is used to write a reference timing pattern. An external head/arm positioner with a displacement measuring device for positional feedback is used to precisely determine transducer location and is the basis for burst placement and spacing of bursts in successive tracks.
FIG. 1A illustrates a conventional “quad” burst servo pattern, see below with reference to U.S. Pat. No. 5,381,281, which includes: (1) an AB servo burst set 20 and 30; and (2) a CD servo burst set 40 and 50. The quad burst servo pattern is written radially within a servo sector and is shown horizontally in FIG. 1A for illustration. The differences between adjacent edges of the A servo burst 20 and the B servo burst 30 define the track center lines and the difference between track center lines defines the track pitch (TP) as shown by 95. Five tracks are shown, i.e., n to n+4. As the read head moves over this servo pattern, position error signals are generated. In the ideal case, position error signals 60 and 70 are generated by this quad burst servo pattern. Position error signals (or “sensitivity signals”) are used to determine the position of the head. For instance, position error signal 60 represents the signals from pattern A minus the signals from pattern B (A−B) and is maximum when the head is in the middle of a burst A and is minimum when the head is in the middle of a burst B. Position error signal 70 represents the signals from pattern C minus the signals from pattern D (C−D) and is maximum when the head is in the middle of a burst C and is minimum when the head is in the middle of a burst D.
In the typical case, gaps between servo bursts and small read heads tend to “round” the tops and bottoms of the ideal position error signals 60 and 70. Therefore, to remain within the linear portions of these signals, the head position control circuit follows a signal 80 that, in effect, switches back and forth between the two signals 60 and 70. The switching occurs when the two signals 60 and 70 have the same magnitude, e.g., at points 90a-90g, thereby avoiding the nonlinear regions located within the tops and bottoms of signals 60 and 70 in the non-ideal case. The first switch of a track occurs at TP/4 and the next occurs at TP/2.
FIG. 1B illustrates a case where the servo bursts are written using a seamless and untrimmed technique, e.g., the positions of burst A and C elements are independent of the positions of burst B and D elements. Because of the independent positioning, seamless writing techniques offer less error than seamed techniques. However, depending on the allowed tolerances, gaps between servo bursts and very small read heads can be expected. As a result of these gaps and due to small read heads, flat or dead zones can appear in the actual position errors signals 60′ and 70′. In some cases, the flat or dead zones appear in both signals 60′ and 70′ at the same head position, e.g., 100a, 100b and 100c. At these positions, the head positioning control circuit is not able to use either of the position error signals to determine the head location because neither signal changes with head position. This can lead to a fatal result for a modern disk drive.
According to U.S. Pat. No. 6,049,442, a servo pattern for use on a data storage surface includes at least one track to minimize position error during positioning of a transducer over the data storage surface. The servo pattern includes a plurality of servo burst fields of constant amplitude for defining a centerline of the track and for determining the position of the transducer. According to this technique, at least one servo burst field comprises N segments, where N is equal to or greater than 2, and each of the N segments is written with constant amplitude. The amplitude of the at least one servo burst field is then determined as a function of the amplitudes of the N segments.
According to U.S. Pat. No. 5,381,281, a quadrature based embedded servo control system is described to realize a high track density, high-performance hard disk drive system. Each data sector includes a gray code field spanning the entire width of the data track and a quad-servo burst pattern having first, second, third, and fourth servo burst fields distributed along the length of a portion of the data sector. The center point of the first, second, third, and fourth servo bursts are sequentially offset from the adjacent burst by a radial distance equivalent to one-half of the data track width. The quad-servo burst pattern is used with a track-following algorithm based on the quadrature value of (A+B)−(C+D) to obtain a substantially increased servo lock range. A second gray code field extending substantially the width of the data track and second quad-servo burst pattern substantially identical to the first is provided near a mid-point in the data portion of the data sector to increase the servoing information sample rate and accuracy, thereby permitting increased data track densities to be utilized.
According to U.S. Pat. No. 5,946,157, a rotating magnetic storage disk drive is described having a method of seamlessly recording circumferentially overlapping servo bursts on a magnetic disk with successive passes of a write head that is guided by a servo track writer wherein the servo bursts are contained in at least two servo burst groups that each have at least one circumferential burst position which may contain a servo burst. The method includes turning a write current on while passing the write head over a current ramp region that does not contain servo data and is located in front of an “active” servo burst group that will be modified on this pass, modifying a servo burst in at least one circumferential position of the active servo burst group with the write current on. The method includes turning the write current off while passing the write head over a current ramp region that does not contain servo data and is located in front of a “passive” servo burst group that will not be modified on this pass and then skipping over at least one circumferential burst position of the passive servo burst group with the write current off.
However, what is needed is a servo mechanism that eliminates the problems associated with non-linearities in the position error signals but also allows seamless and untrimmed servo burst writing techniques which are better than above described art.